diff --git a/lib/fibonacci.rb b/lib/fibonacci.rb
index 606755b..4addfa8 100644
--- a/lib/fibonacci.rb
+++ b/lib/fibonacci.rb
@@ -5,8 +5,25 @@
 # ....
 # e.g. 6th fibonacci number is 8
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n) where n is the number of the input. As n increases, the while loop operations will increase too.
+# Space complexity: O(1). Thought there is an array to store previous_num and this num, the arrays doesn't increase when n increases.
 def fibonacci(n)
-  raise NotImplementedError
+  raise ArgumentError if n.class != Integer
+  raise ArgumentError if n < 0
+  return 0 if n == 0
+  return 1 if n == 1
+  this_num = 1
+  previous_num = 0
+  index = 0
+
+  while index < n - 1
+    array = []
+    array << previous_num
+    array << this_num
+    sum = array[0] + array[1]
+    previous_num = this_num
+    this_num = sum
+    index += 1
+  end
+  return sum
 end